1. Field of the Invention
The present invention relates to a radome that accommodates a radar, and more particularly the invention relates to a radome that is installed in an aircraft, a vehicle, or the like, and that has an aerodynamic shape.
2. Description of the Related Art
With recent improvements in communication technology and information processing technology, a technology for two-way communicating from an aircraft, a vehicle, or the like is being placed in practical use. Particularly for the aircraft, in order to communicate from an installed antenna system therein through the medium of satellites, a wider beam scanning range than the conventional is demanded. Therefore, it is required of the radome that the loss of an electromagnetic wave caused by the reflection of the wave input and output by the antenna be small on the wall of the radome over the wider range of the antenna scanning angle.
In a radome having an aerodynamic shape providing a small resistance to the air in contrast to a ground radome having a hemispherical shape, the angle of incidence of the electromagnetic wave on the wall of the radome is not uniform. In general, when the electromagnetic wave impinges on the wall of the radome at a large angle to the wall, the loss thereof is large. Accordingly, in order to lower the loss of the electromagnetic wave input and output through the antenna at a wider scanning angle of the antenna, it is requested that the loss of the electromagnetic wave occurred through the wall of the radome be small over the wider range of the angle of incidence. A radome used for an aircraft, for instance, is usually produced such that the radome has a sandwich structure obtained by placing a core portion (material) between skin portions (materials) and laminating these materials. For instance, “The Handbook of Antenna Engineering” (edited by IEICE (The Institute of Electronics, Information and Communication Engineers), published by Ohmsha, Oct. 30, 1980, pp. 301) describes a radome conventionally produced by sandwiching and binding a core portion having a low relative dielectric constant between skin portions having a high relative dielectric constant in order to reduce the loss.
By the way, it is required of the radome aboard an aircraft that its dielectric characteristics and mechanical strength for withstanding aerodynamic force be mutually compatible. From this viewpoint, U.S. Pat. No. 5,936,025, for instance, discloses a technology that uses a composite material consisting of a ceramic powder and a resin, limited by a mixture of TiO2 and a cyanate resin in order to adjust the dielectric characteristics of the radome.
A radome having an aerodynamic shape has the property that, when the scanning angle of the antenna changes, the angle of incidence of the electromagnetic wave on the wall of the radome changes far more than the change of the scanning angle. Therefore, in the radome having an aerodynamic shape, constructed according to a conventional technology, there is the problem that the transmission loss extremely increases when the antenna operates at a certain scanning angle, thereby reducing the performance of the antenna.
Moreover, in the radome having an aerodynamic shape, fabricated by a conventional technology, since the transmission loss changes as the angle of incidence changes, the axial ratio of the antenna is disadvantageously deviated. These problems have caused a large increase of cost in antenna designs, and they have simultaneously reduced the performance of an antenna.